1. Field of the Invention
This invention relates to optical communication fiber connectors, and in particular to connectors for a plurality of optical fibers which are stabilized by a substrate.
2. Description of Related Art
The efficient movement and management of information now requires local area networks within buildings and wide area networks across the nation which provide on-line access to data bases and communications services. The concept of an information highway is developing into hardware which will provide wide bandwidth services to every home. Optical fibers provide great bandwidth which is capable of distributing multimedia services in a noise-free and secure network. The distribution of these services will take place in the loop plant of local telephone companies, or others in competition with them, to connect nearly every home and office to the information highway.
A variety of optical fiber designs already exist together with connectors for them. The fibers may be designed for single-mode or multi-mode transmission with cladding or outer diameters of 125 microns and core diameters of approximately five microns or 65 microns, respectively (one micron equals one millionth meter). Other core and cladding diameters are feasible.
There are coupling losses when an optical fiber needs to be connected to another optical fiber or to an opotelectronic apparatus. A permanent connection whereby the glass fibers are butt-fused together with a flame is impractical for most applications. For non-permanent connections, losses in the connection arise from misalignment of the two axes by being off-axis with one another, by having their axes at an angle to one another, by having their ends separated by too great a distance, or any combination of these three. Another possibility for loss is the mismatch between the refractive index of the fibers and an intervening air gap which is left between the ends of the fibers. This loss is often minimized by an index-matching gel or silicone fluid which fills the gap.
The requirements placed upon the connector are, therefore, to locate the axes within plus or minus 0.5 microns or plus or minus two microns of each other, depending upon the mode, and to maintain angular orientation of the two axes to within a few degrees. These tolerances are required to keep the loss per connection to less than a few tenths of a decibel. These tolerances become tighter as the number of connectors in series increases.
Micron type tolerances are common in semiconductor processing, and there is an embedded base of fiber optic connectors which take advantage of semiconductor fabrication tolerances and the ability to etch grooves into silicon to form V-groove connectors which keep an array of optical fibers accurately spaced apart. In addition to accurate spacing, the silicon member provides dimensional stability because it has a low coefficient of thermal expansion and because it is insensitive to moisture. Both of these properties are important in the environment of the loop plant where temperatures can range from -40 degrees Fahrenheit to 160 degrees in a cable vault, and where the connector could be under water. So there is a well working etched silicon connector technology, but it is very expensive when considering all the connectors in series from a source to the final piece of apparatus.
Naturally, attempts have been made to reduce this cost by using the precision of an etched silicon master to electroform a molding die for plastic connectors. Their construction consists of a pair of ferrules with a gum boot, two guide pins that align the ferrules to each other, and a clamp spring which holds the assembly together. Reliance upon the guide pins penetrating two plastic bodies for precise location lowers the coupling efficiency, and therefore the yield, for single mode fiber connections. The thermal expansion of the plastic body, typically 16 parts per million per degree Centigrade, does not match the embedded base of etched silicon connectors whose expansion is 2.3 ppm/deg. C. These connectors do lower cost, typically by 50%, but they are susceptible to dimensional changes as both temperature and humidity cycle in the conditions which are typical in the loop plant. The release of residual molding stresses may also cause warp in the plastic over time.
Accordingly, there is an increased need for a moderate cost optical fiber connector which precisely locates the axes of the fibers and which maintains that alignment in hostile environmental conditions. Additionally, the connector should more closely match the thermal expansion of the embedded base of etched silicon connectors.